The present invention relates to casting of molten metallic material in an investment mold with an exothermic material placed on the molten metallic material subsequent to introduction of the molten metallic material in the mold.
Exothermic material has been employed in the casting art to provide supplemental heat to molten metallic material present in a riser of a sand mold while the molten metallic material in one or more mold cavities solidifies. The molten metallic material in the riser is fed to the solidifying casting(s) in the mold cavities to avoid shorts and other void-type defects resulting from casting shrinkage and lack of adequate supply of molten metallic material during solidification. U.S. Pat. Nos. 2,295,227 and 3,467,172 describe sand molds having exothermic material placed in the sand mold prior to casting of a molten metallic material, such as steel therein.
Exothermic material has been used in making castings wherein molten metallic material is cast into a ceramic investment shell mold made by the well known lost-wax process. For example, a ceramic investment shell mold is formed having a primary or secondary frusto-conical pour cup that includes pre-existing top opening to receive exothermic material on the molten metallic material after it is cast into the mold up to the level or height of the primary or secondary pour cup. This technique limits the locations where exothermic material can be applied on the molten metallic material in the mold and the effectiveness of the exothermic material in making large castings. This technique is further disadvantageous in that a significant excess of molten metallic material is required to fill the primary or secondary pour cup than is necessary to make the casting(s). Moreover, if a secondary pour cup is provided on the mold to receive exothermic material, the additional pre-existing opening (open secondary pour cup) provides an additional potential source for foreign material, such as inclusions, to enter the mold as it handled prior to casting. In addition, the technique is not applicable to investment shell molds that include an inverted loop feedgate of the type described in U.S. Pat. No. 6,019,158 where the molten metallic material is caused to flow upwardly under pressure from a pour cup reservoir through an inverted feedgate passage into the mold cavities.
It is an object of the present invention to provide method and apparatus for casting of a molten metallic material in an investment mold in a manner that exothermic material can be applied on the surface of the molten metallic material subsequent to its being introduced into the mold.
The present invention provides a method as well as investment mold for casting of molten metallic material wherein a refractory shell mold includes a first opening, such as for example a pour cup opening, to receive a molten metallic material to fill a mold cavity. Molten metallic material is introduced into the shell mold through the first opening to fill the mold cavity and provide an upper surface of the molten metallic material in the mold below an initially closed, destructible region of the mold. The initially closed, destructible region is destroyed to provide an entry opening into the mold through which an exothermic material is placed on the upper surface of the molten metallic material in the mold to provide a source of molten metallic material above the mold cavity to be fed thereto to accommodate shrinkage of a casting as it solidifies in the mold cavity.
In an embodiment of the invention, the molten metallic material is introduced into the shell mold such that the upper surface of the molten metallic material resides in a reservoir disposed below an open pour cup. The destructible region of the mold is communicated to the reservoir. In another embodiment of the invention, the destructible region can comprise a blind riser that is communicated to the reservoir and that is broken off after the molten metallic material is cast into the mold.
The present invention aids in feeding of one or more mold cavities with molten metallic material heated by the exothermic material after the mold is cast with less molten metallic material than is required to fill the mold to the level of the pour cup. The invention can be practiced with a variety of investment shell molds including those having an inverted loop feedgate. The entry opening into the mold is made only after the molten metallic material is introduced into and fills the mold cavities.